Recently, advanced high strength steel (AHSS) having a tensile strength of 490 MPa or greater, examples of which include twining induced plasticity (TWIP) steel and transformation inducted plasticity (TRIP) steel, has been developed.
However, when such AHSS is annealed, Si/Mn/Al contained therein may be diffused to the surface thereof to form an Si/Mn/Al oxide layer having a thickness of several tens to several hundreds of nanometers (nm). Si/Mn/Al oxides may prevent the adhesion of molten zinc to AHSS during hot-dip galvanization to cause plating faults.
FIGS. 1 and 2 are a surface analysis graph and a depth profile graph of TWIP steel. Referring to FIGS. 1 and 2, Mn oxides and Al oxides that prevent the adhesion of zinc are formed on the surface of the TWIP steel.
FIGS. 3 and 4 are a surface analysis graph and a depth profile graph of TRIP steel. Referring to FIGS. 3 and 4, Si oxides and Mn oxides are formed on the surface of the TRIP steel. Like TWIP steel, TRIP steel is also difficult to galvanize due to the presence of Si oxides and Mn oxides.
Zinc plating protects steel electrochemically. Non-galvanized steel has a low degree of corrosion resistance, and thus it is difficult to use non-galvanized steel as a material for exterior frames. Even high strength steel such as TWIP steel and TRIP steel is also difficult to use as a material for exterior frames if not galvanized.
Therefore, it is necessary to develop techniques for removing Si/Mn/Al oxides from AHSS. Various attempts have been made to remove Si/Mn/Al oxides from the surface of AHSS or to prevent the formation of Si/Mn/Al on the surface of AHSS, but there is, as yet, no result.
Particularly, since Si/Mn/Al oxide films are formed on the surface of AHSS in the form of islands or networks with a large amount of bonding strength therebetween, it is difficult to remove such Si/Mn/Al oxide films from AHSS.